A major objective of this application is to obtain a better understanding of the interactions between nucleic acids and proteins and of related biological activities including the regulation of gene expression and the mechanisms of DNA replication, transcription and recombination. Toward this end crystallographic, biochemical and mutagenic studies will be pursued of a number of DNA- and RNA-interacting proteins including the exonuclease from phage lambda, exonuclease I from E. coli, the biotin repressor from E. coli, RNA splicing proteins from chloroplasts, proteins involved in asymmetric cell division in Drosophila and RNA-interacting DEAD box proteins. Studies will also be continued of E. coli beta-galactosidase, E. coil F1 ATPase and E. coli methionine aminopeptidase. We also hope to use the beta-galactosidase system to better understand the changes in protein structures that occur on flash-freezing and, at the same time, to develop systematic methods to avoid crystal cracking and other types of damage that often frustrate the use of this technique. The way in which proteins interact with DNA is fundamental to the correct regulation and growth of every living cell. Many human diseases have their origins in a breakdown in these basic control processes. Studies of the sort described here provide fundamental insights into the way in which proteins recognize their target sites on DNA and exert their control functions. There is every reason to expect that understandings derived from such systems in simple organisms will be relevant to human health. Knowledge of the three-dimensional structure of E. coli beta-galactosidase is expected to lead to improvements in a number of the assays that are based on the unique properties of the enzyme.